Motor learning is the process by which movements become smooth and accurate through practice. Motor learning depends on a brain region called the cerebellum, and patients with cerebellar dysfunction have clumsy, uncoordinated movements. This project examines the error signals that guide motor learning. Which neurons in the cerebellum or related neural circuitry monitor the accuracy of a movement, and determine when the motor program controlling that movement needs to be updated? For decades it was thought that the climbing fiber input to the cerebellum from the inferior olive provided the sole instructive signal guiding motor learning. However, we recently showed that motor learning could be induced under training conditions that elicit no response in the climbing fibers. The proposed experiments analyze this climbing fiber-independent component of motor learning. In particular, we will examine the origin of the instructive signals controlling this component of learning, and we will determine which aspects of movement it can and cannot regulate. This project capitalizes on the experimental tractability of eye movements, in particular the vestibulo-ocular reflex (VOR), to analyze the neural mechanisms of motor learning. The VOR is a reflexive eye movement that reduces motion of visual images on the retina by evoking eye movements in the opposite direction to head movements. A form of motor learning, known as VOR adaptation, calibrates the VOR by gradually correcting the reflex when image motion is persistently associated with head turns. VOR adaptation is essential for ensuring adequate visual acuity during head turns and for restoring proper motor and perceptual orientation in space in response to changes in the organism or its environment, such as occur with growth and development, aging, injury to the peripheral or central nervous system or the donning of a new pair of spectacles.